In a conventional CMOS (Complementary Metal Oxide Semiconductor) image sensor, a photodiode is formed over a silicon substrate, and wiring is formed over this photodiode. However, from the viewpoints of improvement in light efficiency and degree of flexibility in design, a backside-illuminated CMOS image sensor has attracted attention, in which light is made incident from an opposite side (i.e., the backside) of a face where a photodiode is formed.
In the backside-illuminated COMS image sensor, a silicon substrate (hereinafter referred to as “device substrate”) over which a photodiode and so on are formed needs to be thinned by mechanical or chemical polishing in order to shorten the distance to the photodiode. When the device substrate is thinned, the device substrate alone cannot withstand polishing pressure. Therefore, the device substrate is thinned after the support substrate is bonded to the device substrate.
However, it is assumed that a conventional backside-illuminated CMOS image sensor undergoes a conventional wafer process after the support substrate is bonded to the device substrate. Accordingly it is easy to imagine that, as a result of metal contamination, a general silicon substrate for use as a support substrate may degrade device characteristics.